1. Field of the Invention
The present invention relates to a substrate used for mass spectrometry, particularly, to a substrate used for mass spectrometry based on the laser desorption ionization method.
2. Description of the Related Art
Mass spectrometry has long been used as one means for identification of substances, particularly organic substances. The principle thereof is to impart a large energy in a short time to a sample to ionize it and analyze the mass of the ion by a detector. As a detector, a quadrupole mass spectrometer, a time-of-flight mass spectrometer and the like are used. Particularly, the time-of-flight mass type detector is recently becoming mainstream. As an energy source to cause ionization, a Curie point pyrolyzer and a laser are used. Although mass spectrometry is favorably used for analysis of low-molecular substances, if an object substance to be measured has a high molecular weight, the original substance has a tendency of being fragmentated and decomposed into relatively low-mass ions. Then, means is proposed to apply the mass spectrometry to detection of materials having a high molecular weight like biomolecules. This means is named Matrix Assisted Laser Desorption Ionization Mass spectrometry (MALDI-MS). This is means which can ionize even a substance having a high molecular weight while preventing the fragmentation by making a mixture of the object substance to be detected and a material named a matrix to be a sample and subjecting the sample to the laser irradiation. Through the spectroscopic method of MALDI-MS, the mass spectrometry has been given attention as evaluation and identification means of biomaterials.
Means is also proposed to conduct mass spectrometry using a porous substrate instead of a matrix material. This method is named Surface Assisted Laser Desorption Ionization Mass Spectrometry (SALDI-MS). Since this method detects no low-molecular peak resulting from a matrix, it is said to be advantageous to the mass spectrometry of relatively low-molecular biomaterials such as metabolites. Techniques using a substrate having a semiconductor porous surface for mass spectrometry are disclosed in U.S. Pat. No. 6,288,390 and U.S. Pat. No. 6,399,177. Particularly a technique related to SALDI-MS using a porous silicon fabricated by anodic conversion of silicon is named Desorption Ionization on Silicon (DIOS), and is disclosed, for example, in Jing Wei, et al., Nature, Vol. 399, pp. 243-246, 1999.
Although MALDI-MS can ionize an analytical object while preventing fragmentation thereof, since the sample is mixed with a matrix material for measurement, MALDI-MS has a problem that mass peaks resulting from the matrix material are observed in large numbers.
On the other hand, since DIOS generates almost no low-molecular MS peaks resulting from a matrix and can prevent fragmentation of a substance to be detected, a high-quality spectrum is obtained.
However, in the case (U.S. Pat. No. 6,399,177) of a porous silicon formed directly on a silicon substrate having a high thermal conductivity, the energy absorbed in the silicon by laser irradiation easily transfers to the silicon substrate. As a result, there arises a problem that the efficiency of the local temperature rise of porous parts carrying a substrate to be detected is reduced.
Therefore, the present invention has an object to provide a substrate for mass spectrometry whose efficiency of temperature rise is enhanced.